This invention is based upon the general problem of simplifying the known preparation of wash-active .alpha.-sulfofatty acid esters from fats and oils, particularly those of natural origin, in such a way that the fatty acids or fatty acid mixtures of vegetable and/or animal origin can be used as a feasible alternative in the production of modern detergent compositions, thus making it reproducible for large scale technical purposes. In the field of washing and cleaning agents, particularly fabric detergents, the emphasis today is, as is known, on petroleum-based synthetic products although it has been known for decades that high-grade wash-active components can also be obtained from, in particular, the fatty acid triglycerides of natural starting materials.
U.S. Pat. No. 2,195,187, incorporated herein by reference, describes wash-active substances (WAS) based upon .alpha.-sulfofatty acids and their esters. They are obtained by sulfonation of lower alkyl esters of saturated higher fatty acids with sulfur trioxide. The lower fatty acid alkyl esters used as starting materials are obtained by transesterification of hydrogenated fats or oils with monofunctional lower alkanols, particularly methanol, or by hydrolysis of the fats or oils, followed by esterification with alkanols.
Later studies with this class of wash-active .alpha.-sulfofatty acids and corresponding fatty acid esters, as well as their salts, have been undertaken. U.S. Pat. No. 3,256,303 incorporated herein by reference, for example, describes a process for the production of this class of compounds. Fatty acids and fatty acid esters, which contain from 6 to 28 carbon atoms in the fatty acid radical, which have no other sulfonatable or sulfatizable groups, apart from the .alpha.-position carbon atom of the fatty acid radical, and which have an iodine number less than 5, are sulfonated with a sulfur trioxide/inert gas mixture, and the reaction product is neutralized. A parallel process for the production of the same compounds, working with alternate process conditions but at the end with the same means, is described in U.S. Pat. No. 3,158,632, incorporated herein by reference.
One of the main difficulties in this field is the poor color-stability of the fatty acid-containing or fatty acid ester-containing starting material in the sulfonation stage. Dark-colored to black-brown crude products are obtained, which products must be worked up into light-colored products for use in washing and cleaning agents. The color of the crude sulfonation products depends to a certain extent on the working conditions. However, the technical utilization of this interesting possibility of the raw material is prevented by the following fact: the higher the yield (degree of sulfonation) is in the sulfonation step, the darker is the reaction product and the greater are the difficulties of obtaining light-colored end products.
Bleaching of the crude sulfonic acid derivatives has always been required as a final process step. Two particular methods are generally known to the art: acid bleaching with hydrogen peroxide (see, for example, U.S. Pat. No. 3,159,657, incorporated herein by reference), and combination bleaching, where an acid hydrogen peroxide bleaching stage is followed by neutralization of the sulfonated and partially bleached material, followed again by bleaching with hydrogen peroxide or, better still, with hypochlorite (see, for example, U.S. Pat. No. 3,452,064, incorporated herein by reference).
Special difficulties, or problems, regarding discoloration occur when the sulfonation is to be increased to yields of over 90% or even to sulfonation degrees of over 95%, which problems are dealt with extensively in U.S. Pat. No. 3,485,856, incorporated herein by reference. According to this patent, sulfur trioxide has a highly disintegrating effect on saturated fatty acid esters which are free of alcoholic hydroxyl groups, which leads unavoidably to very darkly discolored sulfonation products in the production of highly sulfonated products with a degree of sulfonation of at least 90%, preferably at least 94%, and particularly at least 96%.
The increase in the degree of sulfonation in these ranges is not only of interest for economic reasons, however, since other factors require such high degrees of sulfonation too. Ester sulfonates with a corresponding low degree of sulfonation lead to difficulties in the conventional production of detergent compositions by spray drying. High pluming values appear in the processing of these ester sulfonates. Furthermore, the degree of sulfonation of ester sulfonates is directly related to undesired by-products formed in this reaction, namely, .alpha.-sulfofatty acids. These compounds, which are present after neutralization as the disodium salt, are poorly water-soluble and are therefore unsuitable as raw material for detergents. Increasing the degree of sulfonation from 90% to 96% in these ester sulfonates causes, for example, a decrease of this undesired by-product from 25% to 16%.
U.S. Pat. No. 3,485,856, which deals with the last-mentioned problem, suggests that to limit the discoloration and to maintain certain temperatures in the sulfonation reaction, water should be introduced into the sulfonation product in such quantities that sulfuric acid is formed from the existing excess sulfur trioxide and the water, the concentration of the H.sub.2 SO.sub.4 being in the range of from about 20 to 100% by weight at the start of the following bleaching phase. For the large-scale technical process, however, new difficulties arise, which represent a considerable risk source. The viscosity of the crude sulfonation product is greatly influenced in the strongly acidic range by even the slightest traces of water. Even the addition of 2% by weight of hydrogen peroxide in the form of a 35% by weight solution--with the quantities of water inevitably introduced therewith--to the crude sulfonation product with a C.sub.16 -C.sub.18 -chain length, leads to a sharp viscosity rise. In a continuous industrial scale process, there will be substantial risk that the pipelines will become blocked. The viscosity rise is particularly critical with an addition of from 1.8 to 2.5% by weight of hydrogen peroxide, based upon the weight of the crude sulfonic acid.
The best bleaching method for crude ester sulfonates so far is based upon use of hydrogen peroxide in the strongly acidic range (pH=0), the bleaching effect being particularly pronounced. However, such a procedure involves the risk of the above-described sudden viscosity increase. With highly sulfonated ester sulfonates, even 2% by weight of hydrogen peroxide is not enough to bring the Klett dye number to required low values. After neutralization of the crude sulfonic acid, it is therefore necessary to bleach again with sodium hypochlorite. A reduction in the amount of the bleaching agent with a simultaneous increase of the bleaching time leads to more unfavorable colors and gives rise to a number of other difficulties: Due to the great viscosity increase produced by the addition of, for example, 2% by weight of hydrogen peroxide to the crude sulfonic acid, it is not possible to obtain paste concentrations higher than 28% by weight of WAS in the neutralization step. In this bleaching method there are also problems with foaming, which is difficult to control on an industrial scale. In particular, the foam introduced into the crude sulfonic acid leads to a further increase in viscosity.
The many difficulties appearing in the various stages of the total process lead according to our present knowledge to a forced compromise between sulfonation and bleaching. The optimum degree of sulfonation obtainable in practice is about 90%.
Co-pending, commonly assigned U.S. patent application Ser. No. 288,769, filed July 31, 1981, incorporated herein by reference, describes a possible method of reducing the discoloration of the end sulfonation product by subjecting the fatty acid ester starting material to be sulfonated to an additional purification step. In this modified process, a fatty acid ester fraction, from which the accompanying fatty acid glycerides have been removed to a residual content of at most about 1% by weight, preferably no more than about 0.3% by weight, based upon the material to be sulfonated, is introduced into the sulfonation step. The elimination of these "accompanying fatty acid glycerides" from the esters of the fatty acids or fatty acid mixtures of natural origin with monohydric alcohols requires at least two distillations of the fatty acid ester fraction to be sulfonated.